1. Field of the Invention
The present invention generally relates to reticle stage calibration in lithographic apparatus.
2. Description of the Related Art
The term “patterning means” as will be employed herein should be broadly interpreted to refer to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” may also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:                (a) a mask: the concept of a mask or reticle is well known in lithography, and it includes reticle types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid reticle types. Placement of such a reticle in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the reticle, according to the pattern on the reticle. In the case of a reticle, the support structure will generally be a reticle table, which ensures that the reticle can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        (b) a programmable mirror array: an example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        (c) a programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a reticle and reticle table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as set forth above. Also, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
Lithographic exposure apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
In current apparatuses, employing patterning by a reticle on a reticle table, a distinction can be made between two different types of machine. In one type of lithographic exposure apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus —commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Because, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
It is to be noted that the lithographic apparatus may also be of a type having two or more substrate tables (and/or two or more reticle tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Among other things, lithographic systems are used in the manufacture of integrated circuits (ICs). As schematically depicted in FIG. 1A, such systems commonly employ a lithographic exposure apparatus 100 to project or expose a circuit pattern residing on a reticle RE onto a target field C on a layer of silicon wafer substrate W via an irradiating projection beam PB. The projection beam PB may encompass different types of electromagnetic radiation including, but not limited to, ultraviolet radiation (UV) and extreme ultra-violet radiation (EUV), as well as particle beams, such as ion beams or electron beams.
In particular, lithographic apparatus 100 includes radiation source LA and radiation system IL for providing projection beam PB, a first object table (e.g. reticle table) RT provided with a reticle holder for holding a reticle RE, and a projection system PL (e.g., lens) for imaging an irradiated portion of the reticle RE onto a target portion C (e.g. comprising one or more dies) of the substrate W. The combination of the reticle RE, reticle table RT, and reticle-related components are commonly referred to as the reticle stage RS. As depicted, lithographic apparatus 100 is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type (with a reflective mask) and, alternatively, apparatus 100 may employ another kind of patterning means, such as a programmable mirror array of a type as indicated above.
Lithographic apparatus 100 further comprises a second object table (e.g., wafer substrate table) WT provided with a substrate holder for holding a wafer substrate W (e.g. a resist-coated silicon wafer). The combination of the wafer substrate W, wafer table WT, and wafer-related components are commonly referred to as the wafer substrate stage WS.
Source LA produces a beam of radiation, which is fed into illumination system (e.g., illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander EX, for example. Illuminator IL may comprise adjusting means AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the reticle RE has a desired cross-sectional uniformity and intensity distribution.
Projection beam PB subsequently intercepts the reticle RE, which is held on a reticle table RT. The reticle table RT and/or the reticle stage RS may contain an actuating mechanism for adjusting the position of the reticle table RT, including height, tilt, rotational, and level positions. Having traversed the reticle RE, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the wafer substrate W. With the aid of the second positioning means (and interferometric measuring means IF), the substrate table WT can be moved accurately (e.g. so as to position different target portions C in the path of the beam PB). Similarly, the first positioning means can be used to accurately position the reticle RE with respect to the path of the beam PB (e.g. after mechanical retrieval of the reticle RE from a reticle library, or during a scan).
In general, movement of the object tables RT, WT will be realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 1A. However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus), the reticle table RT may just be connected to a short stroke actuator, or may be fixed.
Lithographic apparatus 100 may operate in two different modes:                (a) step mode: reticle table RT is kept essentially stationary, and an entire reticle image is projected in one go (i.e. a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; and        (b) scan mode: essentially the same scenario applies, except that a given target portion C is not exposed in a single “flash”. Instead, reticle table RT is movable in a given direction (the so-called “scan direction”, e.g. the y direction) with a speed v, so that projection beam PB is caused to scan over a reticle image. Concurrently, substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution.        
Regardless of the mode employed, the imaging quality of lithographic exposure apparatus 100 is contingent on the accuracy with which the reticle image is focused on wafer substrate W. An important factor influencing the accuracy of focused image is the deviation of the reticle stage RS from perfect flatness. FIG. 1B provides an example of reticle stage RS unflatness (RSU). Because the height and/or tilt deviations due to reticle stage RS unflatness locally change the effective angle of incidence of the illumination beam on the reticle RE, the XY position of the image features on the wafer W also change, thereby introducing deformities in, and compromising the accuracy of, the resultant image.
To complicate matters further, as indicated in FIG. 1B, the reticle RE is also subject to such deformations. In an effort to mitigate distortions due to reticle stage RS deformations, various methods have been developed to calibrate the reticle stage RS. Such methods involve calibrating the reticle stage RS, which also includes the effects of the reticle RE deformations.